The invention relates to a screen contact assembly for a cathode ray tube and particularly to a structure for bridging a frit seal joining a faceplate to a funnel portion of such a tube.
One piece envelopes for cathode ray tubes are well known in the art; however, such envelopes pose problems that are not present in envelope assemblies comprising a funnel portion and a separate faceplate. For example, when an aluminum layer is evaporated on a phosphor screen of a one piece envelope it is necessary to position the aluminum evaporators a few inches from the phosphor screen in order to direct the aluminum onto the screen surface. At such a close distance the aluminum evaporation tends to be nonuniform thus providing a much thicker aluminum coating on the central portion of the screen, which is close to the evaporators, than at the edge of the screen. This aluminum nonuniformity decreases the effective phosphor efficiency of the central portion of the screen by absorbing energy from the incident electron beam. Furthermore, at the close evaporation distance described above, aluminum particles are sputtered from the evaporators and cling to the surface of the screen thus causing spot rejects.
It is therefore desirable to fabricate and inspect the faceplate of the cathode ray tube, including deposition of the aluminum layer onto the phosphor screen, before sealing the faceplate to the funnel portion of the envelope assembly. This can be done only if the phosphor screening and aluminizing operations are performed on a faceplate which is separate from the funnel and which is then sealed, for example, by frit sealing, to the funnel portion of the envelope. However, frit sealed structures pose problems in that it is necessary to electrically connect the aluminum layer which overlies the phosphor screen to a source of electrical potential.
It is known in the art to connect the aluminum layer on the phosphor screen to the conductive coating on the inside of the funnel, for example, by painting a conductive stripe or evaporating a conductive material over the frit seal; however, such structures are unreliable especially where the frit seal is reentrant causing the conductive striping or coating to become discontinuous. In applications where peak electrical currents in the 2 to 3 milliampere range are experienced, it is also necessary to have an electrical connecting structure which can withstand such high currents.
U.S. Pat. No. 3,432,803 to Nice, issued Mar. 11, 1969, shows a "through the frit" structure providing electrical contact between an external high voltage lead and the aluminum layer on the phosphor screen of the cathode ray tube. U.S. Pat. No. 3,898,501 to Davis et al. issued Aug. 5, 1975, also shows a "through the frit" type of connection. Both the Nice and the Davis et al. structures introduce mechanical stresses in the frit seal area and thus are unreliable structures for providing the proper voltage to the screen of the cathode ray tube.
U.S. Pat. No. 3,876,899 to Davis et al. issued Apr. 8, 1975, discloses an electrical connective member which bridges the frit seal of the cathode ray tube and contacts the aluminum layer that extends along a longitudinal portion of the sidewall of the faceplate assembly. The connective member is disclosed to have a forward contact area formed to have a substantially reverse-turned leading edge or turned-under rounded terminal end having a thickness of about twice the thickness of the body of the conductive member. The rounded turned-under terminal end is located to effect slidable pressured contact to the aluminum layer which forms a tab extending along the sidewall of the faceplate assembly.
The connective member of the Davis et al. U.S. Pat. No. 3,876,899 must be carefully formed to insure that it does not contact the viewing portion of the faceplate since the rounded turned-under terminal end of the connective member is non yielding in the longitudinal direction and would rupture the aluminum layer on the viewing surface of the phosphor screen.
In cathode ray tubes such as projection tubes, the faceplate is sealed directly to the funnel portion of the envelope. In such tubes there is no longitudinally extending sidewall portion of the faceplate and thus the aluminum layer on the screen terminates adjacent to the sealing edge of the faceplate. The Davis et al. connective structure cannot be used in such a tube since the rounded terminal end of the connective member would rupture the aluminum screen coating and break the electrical connection to the screen.